The present invention generally relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention relates to a coating system that inhibits the formation of deleterious phases in the surface of a superalloy that is prone to coating-induced metallurgical instability.
Certain turbine, combustor and augmentor components of gas turbine engines are susceptible to damage by oxidation and hot corrosion attack, and are therefore protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat. In combination, the TBC and bond coat form what has been termed a TBC system.
Environmental coatings and TBC bond coats in wide use include diffusion coatings that contain aluminum intermetallics (predominantly β-phase nickel aluminide (beta-phase NiAl) and platinum aluminides (PtAl)), and overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth metals, and/or reactive metals). Other types of environmental coatings and bond coats that have been proposed include beta-phase nickel aluminide (NiAl) overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions (such as γ-Ni) containing intermetallic phases (such as beta-phase NiAl), beta-phase NiAl overlay coatings are predominantly the beta-phase NiAl intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Examples of beta-phase NiAl overlay coatings are disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, U.S. Pat. No. 6,291,084 to Darolia et al., and U.S. Pat. No. 6,620,524 to Pfaendtner et al. The suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain the gamma-prime phase (γ′-Ni3Al) has also been considered, as disclosed in U.S. Patent Application Publication Nos. 2004/0229075 to Gleeson et al., 2006/0093801 to Darolia et al., and 2006/0093850 to Darolia et al. Aside from use as additives in MCrAlX overlay coatings, diffusion aluminide coatings, and gamma-prime phase NiAl coatings, platinum and other platinum group metals (PGM's) such as rhodium and palladium have been considered as bond coat materials. For example, commonly-assigned U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses PGM-based diffusion bond coats formed by depositing and diffusing platinum, rhodium, or palladium into a substrate surface, or alternatively diffusing a PGM into an otherwise conventional bond coat material.
TBC systems and environmental coatings are being used in an increasing number of turbine applications (e.g., combustors, augmentors, turbine blades, turbine vanes, etc.). The material systems used for most turbine airfoil applications comprise a nickel-base superalloy as the substrate material, a diffusion platinum aluminide (PtAl) as the bond coat, and a zirconia-based ceramic as the thermally-insulating TBC material. A notable example of a PtAl bond coat composition is disclosed in U.S. Pat. No. 6,066,405 to Schaeffer. Yttria-stabilized zirconia (YSZ), with a typical yttria content in the range of about 3 to about 20 weight percent, is widely used as the ceramic material for TBC's. Improved spallation resistance can be achieved by depositing the TBC by electron-beam physical vapor deposition (EB-PVD) to have a columnar grain structure.
Approaches proposed for further improving the spallation resistance of TBC's are complicated in part by the compositions of the underlying superalloy and interdiffusion that occurs between the superalloy and the bond coat. For example, the above-noted bond coat materials contain relatively high amounts of aluminum relative to the superalloys they protect, while superalloys contain various elements that are not present or are present in relatively small amounts in bond coats. During bond coat deposition, a “primary diffusion zone” of chemical mixing occurs to some degree between the coating and the superalloy substrate as a result of the concentration gradients of the constituents. At elevated temperatures, further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. The migration of elements across this interface alters the chemical composition and microstructure of both the bond coat and the substrate in the vicinity of the interface, causing what may be termed coating-induced metallurgical instability, often with deleterious results. For example, migration of aluminum out of the bond coat reduces its oxidation resistance, while the accumulation of aluminum in the substrate beneath the bond coat can result in the formation of topologically close-packed (TCP) phases that, if present at sufficiently high levels, can drastically reduce the load-carrying capability of the alloy. These detrimental effects occur whether the coating is used as a bond coat for a TBC, or alone as an environmental coating.
Certain high strength superalloys contain significant amounts of refractory elements, such as rhenium, tungsten, tantalum, hafnium, molybdenum, niobium, and zirconium. If present in sufficient amounts or combinations, these elements can reduce the intrinsic oxidation resistance of a superalloy and, following deposition of an aluminum-containing coating, promote the formation of a secondary reaction zone (SRZ) in which deleterious TCP phases form. An example of such a superalloy is commercially known as MX4, a fourth generation single-crystal superalloy disclosed in commonly-assigned U.S. Pat. No. 5,482,789 and exhibiting superior intrinsic strength relative to earlier-generation single-crystal superalloys. Other notable examples of high-refractory superalloys include single-crystal superalloys commercially known under the names René N6 (U.S. Pat. No. 5,455,120), CMSX-10, CMSX-12, and TMS-75, each of which has the potential for being prone to SRZ.
Significant efforts have been put forth to control SRZ in single-crystal superalloys. For example, commonly-assigned U.S. Pat. Nos. 5,334,263, 5,891,267, and 6,447,932 propose direct carburizing or nitriding of a superalloy substrate to form stable carbides or nitrides that tie up the high level of refractory metals present near the surface. Other proposed approaches involve blocking the diffusion path of aluminum into the superalloy substrate with a diffusion barrier coating, examples of which include ruthenium-based coatings disclosed in commonly-assigned U.S. Pat. No. 6,306,524 to Spitsberg et al., U.S. Pat. No. 6,720,088 to Zhao et al., U.S. Pat. No. 6,746,782 to Zhao et al., and U.S. Pat. No. 6,921,586 to Zhao et al. Still other attempts involve coating the surface of a high rhenium superalloy with chromides or cobalt prior to aluminizing the surface, as disclosed in U.S. Pat. No. 6,080,246. Finally, above-noted U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses that a PGM-based coating diffused directly into a superalloy substrate can eliminate the need for a traditional aluminum-containing bond coat and thereby avoid SRZ and TCP phase formation.
Notwithstanding the above, there are ongoing efforts to develop coating systems that substantially reduce or eliminate the formation of SRZ in high-refractory alloys.